This invention relates to the field of AM Stereo tone detection and, more particularly, to the prevention of false or unsatisfactory operation of a stereo tone detector and other stereo receiver circuits due to reception of an intefering signal.
Co-channel interference; i.e., interference caused by another transmitter at almost the same frequency as the one being listened to, is well known in the field of broadcasting. Such interference can be a problem for monophonic receivers in that the two audio signals would be heard simultaneously. In an AM stereo receiver, it can cause additional problems. In some stereo receivers, a phase locked loop (PLL) is used to derive a cosine correction signal from the phase modulated carrier (RF or IF). In other receivers, the PLL may also be used in the demodulation process to provide the stereophonic audio output signals, usually termed L and R for the left and right signals of a stereo program. In the case of the cosine correction signal, co-channel interference may cause a very slight amount of distortion, but the effect on the demodulation of L and R can be an apparent side-to-side movement of a signal source. If the signal which indicates the presence of stereophonic transmission is an infrasonic tone; e.g. 5-25 Hz, co-channel interference may cause false "stereo detect" signals when, in fact, no stereo signal is being received. These detect signals could not only indicate the presence of stereo as by a "stereo" light but can enable the stereo mode of operation, an undesirable effect. If stereophonic signals with the stereo tone added are being received when co-channel interference occurs, the receiver would remain in stereo but the apparent motion of the signal source would be perceived. If co-channel interference is significant, therefore, it would be desirable to disenable the stereo mode of operation. In some broadcast situations, adjacent channel interference may also be a problem, also making the monophonic mode more desirable.